FIG. 11 shows a conventional plasma doping device generally indicated by reference numeral 200. The device 200 has a container 202 defining a vacuum chamber 204 therein and an electrode or table 206 provided within the chamber 204 for supporting a substrate 208. The container 202 is connected to both a gas supply 210 for supplying a doping gas such as B2H6 and a vacuum pump 212 for generating a vacuum in the chamber 204. Also provided to the container 202 are a microwave guide 214 which emits micro wave through a window 216 into the chamber and a magnetic device 218 for directing the micro wave toward the substrate 208. The window 216 is made of a dielectric material such as silica glass. Also, the table 206 is connected through a capacitor 220 to a high frequency power source 222 for controlling the voltage of the table 206 and thereby an amount of impurity to be doped in the substrate. In operation, the doping gas is supplied into the chamber 204 where it is ionized by the interaction between the micro wave and a DC magnetic field to form a microwave plasma, i.e., cyclotron resonance plasma 224. Then, the ionized boron is implanted in the surface of the substrate 208 with an aid of the power source 222. For example, the substrate 208 is then formed with a metal wiring layer on the doped surface. In addition, a thin oxidation layer is generated on the metal wiring layer. Finally, gate electrodes are formed on the surface by a conventional deposition technique such as CVD, which results in MOS transistors.
However, it has been understood that the doping gas such as B2H6 including boron which exhibits an electric activity when it is added to the substrate such as a silicon substrate is a toxic material. Also, according to the plasma doping, all the materials in the doping gas are doped in the substrate. For example, in a case of B2H6, although boron is the only effective material, not only boron ion but also hydrogen ion is doped in the substrate. Then, the doped hydrogen ion can result in a generation of a lattice defect in the substrate at the subsequent heat treatment such as an epitaxial growth process.
To overcome this problem, there has been proposed another doping device in JP 9-115851 (A), which is shown in FIG. 12. The doping device generally indicated by reference numeral 230 has a block 232 including an impurity and is provided within the chamber 204. The block 232 is supported by a fixed support 234 electrically connected through a capacitor 236 to a high frequency power source 238. With this arrangement, a gas such as argon fed from the gas supply 210 is ionized to form the plasma ion, which in turn impinges against the block to draw the impurity ion therefrom for its implantation into the substrate. This device certainly eliminates the drawbacks of the previous device in FIG. 11, however; it renders the device bulky due to the additional structures including support 234. Also, the impurity ion drawn from the block 232 is implanted unevenly in the substrate 208 due to the unsymmetrical arrangement of the block 232 and the substrate 208.